1. Field of the Invention
The present invention relates to a control device for a V belt type continuously variable transmission provided with a torque converter.
2. Description of the Prior Art
A V belt type of continuously variable transmission for a vehicle is known into which the output from the engine is input, via a torque converter and that continuously changes the transmission gear ratio using variable pulleys and a V belt.
In this type of continuously variable transmission, the target transmission gear ratio is calculated in accordance with the vehicle speed and throttle opening that are based on the depressing of the accelerator pedal by the driver, and the oil pressure to the variable pulleys is controlled such that the actual transmission gear ratio reaches the target transmission gear ratio.
More specifically, by controlling the oil pressure to the variable pulley on the drive side, the width of the contact between the V belt and the respective variable pulleys on the drive side and driven side is changed resulting in the gear changing being performed.
The contact friction force between the V belt and the variable pulleys is controlled in accordance with the line pressure. This line pressure is set so as to correspond to the input torque input into the continuously variable transmission. If the line pressure is excessively low relative to the input torque, then slippage takes place between the V belt and the variable pulleys. It the line pressure is excessively high, then the drive loss of the oil pump and the like increases.
Here, because a torque converter is provided between the continuously variable transmission and the engine, it is not possible for the aforementioned input torque to be detected directly. Therefore, as is shown in FIG. 8A, firstly, the input revolution speed of the torque converter (=the revolution speed Ne of the engine) and the output revolution speed of the torque converter (=the revolution speed Np of the primary pulley) are measured. Next, as is shown in FIG. 8B, the speed ratio (output revolution speed/input revolution speed) xcfx81 is determined. Next, the input torque that is input into the continuously variable transmission is estimated from the relationship between the torque ratio of the torque converter and the speed ratio such as that shown in FIG. 9 that is set in advance. The target line pressure is set and the line pressure is appropriately controlled so as to correspond to this estimated input torque.
Generally a large number of revolution speed sensors are provided of the type that detect the revolution speed from the time intervals between detections of teeth formed on the outer circumference of the revolving body and these are commonly used for the measurements of the input revolution speed Ne or the output revolution speed Np that form the foundation for determining the speed ratio of the torque converter.
However, the teeth pitches have become larger and the accuracy of the revolution sensor lowered due to the increasingly smaller and more integrated structure that have been the result of the progressively higher levels of required compactness for the torque converter and continuously variable transmission as a whole. As a result, the line pressure at the time when the accelerator pedal is depressed from a completely closed state is insufficient causing the belt to slip.
Namely, if the accuracy of the measurement of the output revolution speed and the input revolution speed is low as shown in FIG. 10A for example, than as is shown in FIG. 10B, the speed ratio xcfx81 is calculated in a step configuration as is shown by the solid line in contrast to the actual values that are shown by the broken line with the width size of the first stop being 0.06 to 0.1 seconds and the size of the next step also being approximately 0.05 seconds. These width of the steps are the times the sensor is dead.
As a result, when the accelerator pedal is initially depressed, the speed ratio for the interval between each step is usually at a higher value than the actual value. Thus, as shown in FIG. 11, in spite of the fact that the actual value is Qr, the measured value Qk of the torque ratio of the torque converter is lower than Qr by the amount of the disparity between the measured values and the actual values, for example, S.
The speed ratio of the torque converter does recover to rise after temporarily dropping abruptly when the accelerator pedal is depressed from a completely closed state, however, the aforementioned disparity amount in the area where the torque converter speed ratio drops abruptly while the engine revolution speed is low directly after the accelerator is depressed is striking.
In this manner, because the torque ratio of the torque converter is determined to be low and the input torque that is input into the continuously variable transmission is estimated to be excessively small, the line pressure is insufficient under control performed to correspond to this input torque, thereby causing the belt to slip.
If a predetermined additional margin value is added to the calculated value Qk of the torque ratio in order to restrain this belt slippage, then the line pressure increases across the entire range causing the fuel consumption to deteriorate and also shortening the life of the belt due to the increased stress being applied to the belt.
Accordingly, in view of these drawbacks, it is an object of the present invention to provide a control device for a continuously variable transmission that accurately corrects the disparity between the measured values and the actual values of the torque ratio of a torque converter and performs the appropriate line pressure control even when the level of accuracy of the revolution speed sensor is low.
Therefore, the first aspect of the present invention is a control device for a V belt type continuously variable transmission for a vehicle, the transmission being connected to an engine via a torque converter, and provided with a variable pulley formed from a primary pulley and a secondary pulley and a V belt, in which a contact width of the V belt with the variable pulley is controlled by a transmission control section that controls oil pressure to the variable pulley so as to continuously change a transmission gear ratio between the primary pulley and the secondary pulley, wherein the transmission control section comprises: an engine torque calculation section for calculating output torque from the engine; an input side revolution speed sensor for detecting a revolution speed on an input side of the torque converter; an output side revolution speed sensor for detecting a revolution speed on an output side of the torque converter; a torque converter speed ratio calculation section for calculating a speed ratio between an input and an output of the torque converter based on the respective revolution speeds detected by the input side revolution speed sensor and the output side revolution speed sensor; a torque ratio calculation section for calculating from the torque converter speed ratio a torque ratio between an input and an output of the torque converter; an input torque estimation section for estimating from the torque ratio and the output torque from the engine an input torque input into the continuously variable transmission; and a line pressure setting section that sets oil pressure for the variable pulley based on the input torque estimated by the input torque estimation section, and wherein the torque converter speed ratio calculation section is provided with a correction amount calculation section for calculating a correction amount by which the input side revolution speed is to be increased in the initial stages of the depressing of the accelerator pedal relative to a simple ratio of the respective revolution speeds detected by the output side revolution speed sensor and the input side revolution speed sensor, and outputs the speed ratio in which the simple ratio has been corrected by the correction amount.
Because the correction is performed in the direction in which the input side revolution speed increases relative to the simple ratio of the respective detected revolution speeds of the output side and the input side of the torque converter, there is no delay relative to the actual change of the speed ratio. As a result, because the torque ratio is no longer determined to be the excessively low value, it is possible to make the optimum estimation of the input torque input into the continuously variable transmission. Also, since the correction to increase the input side revolution speed is made in the area of initial stage of the depressing of the accelerator pedal, the fuel consumption performance and durability are improved compared with when a margin value is added uniformly.
According to the second aspect of the present invention, the correction amount calculation section calculates a correction amount proportional to the inverse of the input side revolution speed, and the torque converter speed ratio calculation section sets as the speed ratio the ratio of the output side revolution speed to a value obtained by adding the correction amount to the input side revolution speed.
According to the third aspect of the present invention, the correction amount calculation section calculates the correction amount as a coefficient of the input side revolution speed, and the torque converter speed ratio calculation section adds the correction amount to the simple ratio of the input side revolution speed and the output side revolution speed and sets this as the speed ratio.
this time, while the input side revolution speed is lower than a predetermined value, the correction amount may be set at a negative fixed value, and when the input side revolution speed becomes greater than the predetermined value, the correction amount can be expressed by a straight line that increases at a predetermined gradient from the fixed value.
According to another aspect of the present invention, the correction amount calculation section calculates the correction amount as a coefficient of the output side revolution speed, and the torque converter speed ratio calculation section adds the correction amount to the simple ratio of the input side revolution speed and the output side revolution speed and sets this as the speed ratio.